1. Field of the Invention
The present invention relates to a resin composition for coating optical fiber, and, more particularly, to a resin composition for coating optical fiber which, after curing, exhibits stable mechanical characteristics at high temperatures and no changes in color, and maintains durability for a long period of time.
2. Description of the Prior Art
Glass fiber used for optical fiber is very fragile and easily damaged. Therefore, coating the fiber with a resin which is curable by ultraviolet (UV) radiation is provided for protection and reinforcement.
Variations over time in mechanical characteristics, such as the Young's modulus and changes in the color of the UV curable resins must be small from the aspect of long-term reliability. This requires the content of noncurable components in the UV curable resin films to be as small as possible.
In order to improve the productivity of optical fiber manufacturing, the drawing speed of melted glass fiber material must be high. The high drawing speed, however, reduces the amount of radiation by UV light during the UV curable resin covering step, thereby leaving a greater amount of uncured components in the UV cured resin.
Therefore, along with the development of resins which satisfy various requirements in a variety of applications of these resins, the studies are ongoing for increasing the curing rate and curing degree. In addition, with regard to the reliability of the resins over a long period of time, various attempts for designing resin compositions having satisfactory properties such as heat resistance and weather resistance for preserving good outward appearance have been undertaken, including the investigations concerning the use of additives. No resin compositions have been developed, however, which are completely satisfactory.
Because UV curable urethane acrylate resins can satisfy a wide variety of requirements, they have been generally used for coating optical fiber. When used as a primary coating material, these UV curable urethane acrylate resins provide a film which contains only a very small amount of uncured components and which exhibits superb stability with little changes in the properties, while maintaining very soft properties. When used as a secondary coating material, they provide a film exhibiting tough properties and relatively hard. When they are used as matrix material, e.g. in ribbon structures, in addition to the characteristics required for the secondary coating material, they satisfy the requirements for good surface properties.
However, coating materials are desired which enable optical fibers to be usable under various conditions, occasionally, under very stringent conditions which have not been experienced by conventional optical fibers.
An object of the present invention is therefore to provide a liquid curable resin composition for optical fibers, which, while maintaining the superior properties and the high productivity of conventional urethane acrylate resins, exhibit excellent durability after curing.
In view of this situation, the present inventors have undertaken extensive studies and found that a resin composition containing a polymer having a specific type (meth)acryl group can produce a film which stably exhibits high mechanical properties at a high temperature with no change in color. The present inventors have found this resin composition to be useful for optical fiber coating.